The term “array antenna with reflector(s)” should be understood here to mean an antenna consisting of a set of feeds (or radiating elements), defining an array, and one or more reflectors.
The abovementioned array antennas with reflector(s) are of particular interest because they make it possible to form and position one or more beams radiating towards one or more given coverages. This beam formation is done by controlling the amplitude and/or phase at the feeds.
The capacity to modify the position and the shape of the coverages in orbit (double reconfigurability) is of particular interest, particularly to take account of the changes in the traffic, to take over from a failed satellite, or on changes of position in the orbital arc with retention of the link budget over a given zone. In order to allow for a double reconfigurability, the three solutions described below are most commonly used.
A first solution consists in using an active array antenna with direct radiation (or DRA), in other words an antenna with no reflector. This type of array antenna offers a very good double reconfigurability capacity, but requires a large number of controls which often make its cost and its weight prohibitive. Furthermore, in transmission, the low efficiency of the amplifiers that are associated with each of the DRA controls induces an often prohibitive dissipation.
A second solution involves using an array of feeds in the focal plane or in the vicinity of the focal plane of a non-shaped parabolic reflector (or FAFR). This solution is described in particular in the document U.S. Pat. No. 4,965,587. In order to cover a given zone, the feed array is dimensioned in such a way that each of its feeds contributes to a part of the total coverage. The positioning of the feeds is directly linked to the zone to be covered. It is determined geometrically by applying the principle of reflection on the reflector. The amplitude/phase laws of the different controls must be optimized for the beams delivered by the feeds to combine and give a radiation pattern suited to each zone to be covered. If only one of the zones, provided initially, is to be covered, only a part of the corresponding array is used. The amplitude range applied to the radiating elements is large, which often makes it necessary, in transmission, to use a power balancing device between the amplifiers (called MPA).
The fact that each of the feeds is directly linked to a part of the coverage, on the one hand, imposes a redundancy on the amplifiers in order to avoid the loss of this zone in the event of a partial loss, and on the other hand, induces a number of feeds (and often of controls) that is directly linked to the size of the coverage. The beam-forming architecture is therefore particularly complex, induces additional losses linked to the presence of the MPA, and results in a fairly high volume and weight.
A third solution, a variant of the second, has been proposed in the US patent document 2004/0222932. It consists in placing a feed array in the focal plane of a reflector, the reflecting surface of which is shaped so as to spread the area covered by each beam having a “flat” radiation pattern in the main lobe delivered by an individual feed. The principle remains the same as that described above, each feed contributing only a part of the coverage. Because of the spreading of the individual beams introduced by the shaping of the reflector, the number of feeds needed to sample the coverage can then be reduced, which makes it possible to reduce the number of antenna controls.